Geometric Properties of the Magnetic Curve. 319 



The divergent magnetic curves are capable of being de- 

 scribed by an instrument of a similar construction to the one 

 already explained ; only the ruler B n, Fig. 6, must be of 

 twice the length of the former ; and in order to obtain a suffi- 

 cient extent of curve, the revolving rulers, N n> and S s, must 

 be prolonged in those parts where the intersections are to take 

 place. 



ON THE FIRST INVENTION OF TELESCOPES, COL- 



LECTED FROM THE NOTES AND PAPERS OF THE 



LATE PROFESSOR VAN SWINDEN. 



BY DR. G. MOLL, OF UTRECHT. 



[Communicated by Professor Moll.] 



PHE late Professor Van Swinden had been at considerable 

 pains to illustrate some important points in the history 

 of natural philosophy. The first invention of telescopes in 

 Holland attracted a considerable share of his attention, and he 

 had the good fortune to meet with some official documents, 

 which are calculated to throw some light on the mystery in 

 which the early history of this celebrated invention is involved. 



Mr. Van Swinden exposed the result of his labours in several 

 public lectures, and he intended to publish a paper on the 

 subject : his death prevented the accomplishment of this 

 purpose. He left, however, the sketches of his lectures, toge- 

 ther with extensive notes, and abstracts from various writers, 

 which he had collected with great industry. These papers 

 were committed to my hands, and the result of what I collected 

 from them has been ordered to be printed by the Royal Insti- 

 tute of the Netherlands. 



The little which is known of the first invention of telescopes 

 in this country has been principally derived from two sources : 

 first, from the book which the French physician, Pierre Borel, 

 wrote on the subject in 1655, probably at the request, and 

 certainly with the assistance, of William Boreel, at that time 

 ambassador of the States at the court of France*. The second 



* De vero Telescopii inventore, cum brevi omnium Conspicilliorum 

 historia, authore Petro Borello, Regis Christianissimi consiliario et 

 medico ordinario; Hagse Comit. ex typogr. Adriani Vlacq. 4to., 1655. 



Y2 



